Modular Composite Floor Units

ABSTRACT

The invention provides a modular composite floor unit and a method for its manufacture. The floor unit is factory-made. An edge frame ( 10 ) is provided from cold-rolled sheet metal members ( 24  and  32 ) welded or brazed together to create edge shuttering. A cast concrete ceiling slab ( 12 ) is cast within the edge frame ( 10 ) over a smooth casting surface. The cast ceiling slab ( 12 ) encases a first inturned lip of the edge frame ( 10 ), a first lattice ( 26 ) of reinforcing rods or wires anchored at their ends to opposite sides and ends of the edge frame ( 10 ), and the bottom edges, or hangers ( 70 ) suspended below the bottom edges, of an array of mutually parallel spaced metal joists ( 18 ) which are welded or brazed to the edge frame ( 10 ) at their opposite ends. An infill layer is then created from blocks ( 16 ) or particulate material filling most of the height of the exposed portions of the array of mutually parallel spaced joists ( 18 ). A concrete floor slab ( 14 ) is cast within the edge frame ( 10 ) over the top of the infill layer, encasing an upper inturned lip of the edge frame ( 10 ), a second lattice ( 28 ) of reinforcing rods or wires anchored at their ends to opposite sides and ends of the edge frame ( 10 ), and the top edges, or anchorage members ( 60 ) secured to the top edges, of the mutually parallel spaced joists ( 18 ). The top surface of the cast floor slab ( 14 ) is float-finished to create a final floor unit that requires no screeding. The bottom surface of the cast ceiling slab ( 12 ) has a finish defined by the surface on which it was cast, and is visible without further treatment as the ceiling of the room below the floor unit when the unit is used in the construction of a multi-storey building.

FIELD OF THE INVENTION

The invention relates to modular composite floor units, being componentsof modular building systems or of steel frame building systems for therapid construction of buildings for use either as industrial orcommercial premises or as dwellings. The invention also relates to amethod for the manufacture of such modular composite floor units.

BACKGROUND ART

Modular buildings can be constructed from prefabricated wall panelswhich are bolted or welded together on site to create the framework ofthe building. The prefabricated wall panels can include pre-installedwindow frames, door frames, electrical connections and/or plumbingconnections to reduce the building and finishing time on-site, and in atypical modular construction process are assembled on-site by beingmoved into position by a crane or other lifting equipment before beingconnected together to create a rigid structure. If the building is asteel-framed building then similarly the girders are lifted intoposition on-site and connected together to create the rigid framework ofthe building onto and into which are secured the desired external andinternal wall panels.

The floors of such buildings can be hollow or solid. By “hollow” floorsthere is conventionally meant floors created from planks or panels,generally of timber or timber-based composite materials such as plywood,chipboard and oriented particle board, laid over a supporting structuresuch as timber joists or metal beams. By “solid floors” there isconventionally meant concrete floors.

Solid floors are often preferred for their better sound insulationproperties, and are often specified for multi-occupancy buildings suchas apartments, hotels and student accommodation and for industrial andcommercial premises. Generally solid floors are made principally fromconcrete or reinforced concrete, which may be poured on-site. The edgesof the solid floor created by pouring wet concrete are defined by thebrick work or block work defining the periphery of the building or theroom within the building into which the floor is being laid, or by edgeshuttering positioned on-site. That edge shuttering may then be removedonce the concrete has set, or may remain in position.

Solid floors may alternatively be created by laying pre-cast concreteflooring panels. Those panels are pre-cast off-site in open moulds andgenerally incorporate metal reinforcement bars. They are often cast withlongitudinal holes or channels to reduce the overall weight, and alsoare often cast with a slight convex shape which assists stressdistribution in the final building. Ultimately however each array ofpre-cast solid flooring panels is covered with a cement screed to smoothout the surface imperfections and irregularities. The screeded area mustbe kept clear of construction personnel while the cement screed driesand sets, and this of necessity slows down the construction processrequiring work on-site to be stopped or diverted to other areas untilthe screed is sufficiently hard and durable to accept foot trafficwithout damage.

EP-A-881067 discloses a modular composite wall or floor; unit and amethod for its manufacture. In fact the strength requirements and inparticular the fire resistance performance specifications for wall andfloor units are vastly different, so the teaching of EP-A-881067 shouldnot be misunderstood as being that a single product can be laidvertically as a wall or horizontally as a floor. The wall and floorunits are substantially different products but according to EP-A-881067can share common design concepts. The following summary of the relevantteachings of EP-A-881067 is therefore restricted to its teachings offloor units only.

The floor unit of EP-A-881067 is a modular floor unit in the sense thatt is cast off-site and then transported to the site of the buildingunder construction. It is a composite floor unit in the sense that it isnot a single cast slab of concrete that would typify a solid floor unit.It is cast as two concrete slabs separated by an air space or by a layerof insulation (thermal and/or acoustic insulation). The two concreteslabs are cast one at a time in a metal form which has a base and sides.The base gives a smooth finish to the underside of the first slab to becast, while the sides of the form create the side shuttering for the wetconcrete of that first slab. A corrugated plate or array of metall-beams is placed over the top of the first slab to be cast, and createsa support surface for the base of the second slab to be cast. The sidesof the second cast slab are defined by the same shuttering as that usedto define the sides of the first cast slab, namely the sides of themetal form. If desired, an edge detail such as a peripheral recess canbe added to the second cast slab by positioning a form liner around theperiphery of the form before casting the second concrete slab. Aftercasting, and after the concrete has set, the cast composite floor unitis lifted out of the form and any form liner removed, to obtain thefinal composite floor unit in which the valleys of the corrugated sheetor the bottom flanges of the I-beam are partially immersed in the setconcrete of the first (bottom) cast slab and the peaks of the corrugatedsheet or the top flanges of the I-beams are partially immersed in theset concrete of the underside of the second (top) cast slab. Thecomposite structure includes a void between the two cast slabs, althoughthat void may if desired be filled with a thermal or acoustic insulationsuch as a foamed resin composition.

Both the thermal and the acoustic performance of the composite floorunit of EP-A-881067 leaves much to be desired. Acoustically, the I-beamsor spans of the corrugated metal sheet connecting the top and bottomcast slabs provide a direct sound path from one cast slab to the other,so the filling of the void with an acoustic insulating material doesvery little to prevent the transmission of sound from the floor definedby the top face or the top slab to the ceiling defined by the bottomface of the bottom slab. Fire resistance is also very poor. In a firsttest, the bottom slab would rapidly detach from the corrugated metalsheet or I-beams, and the structural integrity of the composite floorunit would soon be lost. The composite floor unit of EP-A-881067 wouldtherefore fall very far short of compliance with British Standard 476,Part 21: 1987, clause 7. That fire resistance standard requires that thestructural integrity of the floor unit should be maintained withinspecified limits even after exposure of one face of the floor unit to afurnace temperature rising to over 1150° C. over a period of 4 hours,and that the mean temperature rise of the face remote from the furnaceshould be no more than 140° C., with a peak temperature rise of no morethan 180° C. Test results are normally reported in terms of the timeduration that elapses before one of the monitored parameters indicatesfailure of the test specimen, either by some loss of structuralintegrity or by an unacceptable temperature rise at the face remote fromthe furnace.

It is an object of the invention to create a modular composite floorunit which exhibits both good thermal and good acoustic insulation andis capable of markedly better performance characteristics than that ofEP-A-881067.

It is desirable that both the upper and lower surfaces of the compositefloor unit are smooth. Therefore without on-site screeding the floorunit will present an acceptably smooth finish suitable for tiling orcarpeting; whereas the underside is preferably smooth enough or has asufficiently accurate surface finish to be visible as a decorativesmooth or patterned ceiling finish to the room below.

Most importantly, however, it is a further object of the invention tocreate a modular composite floor unit which can meet the fire resistanceperformance demands of British Standard 476, Part 21: 1987, clause 7.

THE INVENTION

The invention provides a modular composite floor unit as defined inclaim 1. The invention also provides a method for the manufacture ofsuch a floor unit, as defined in claim 26.

One feature of the floor unit of the invention that is not found in thefloor unit described in EP-A-881067 is that according to the inventionthe edge frame forms a permanent part of the floor unit, whereasaccording to EP-A-881067 it is a temporary form from which the floorunit is removed prior to use. The edge frame of the floor unit of theinvention is welded or brazed to the ends of the lattice of reinforcingrods which ultimately will reinforce the material of the ceiling slab.Also the spaced metal joists which take the weight of the two cast slabsare, according to the invention, welded or brazed at their ends to themetal of the edge frame. The result is a composite floor unit whichconsiderably outperforms that of EP-A-881067 in fire resistance tests,and which can survive the test of BS 476, Part 21: 1987, clause 7 forthe full 4 hours of the test duration without failure. At first itappeared desirable to weld or braze to the edge frame the lattice ofreinforcing rods which ultimately will reinforce the material of theflooring slab. Surprisingly however it has been found that the aboveexcellent fire resistance is obtained when only the reinforcing rods ofthe cast ceiling slab are welded or brazed to the edge frame, and thereinforcing rods of the cast flooring slab are free from the edge frame.Freeing the ends of the reinforcing rods of the flooring slab in thisway makes it possible for the flooring slab to be constructed as afloating floor, which gives the composite floor unit of the inventionreally outstanding acoustic insulation properties. Although fireresistance could in theory be improved further by connecting the ends ofthe flooring slab reinforcing rods to the edge frame, this would be atthe expense of increased sound transmission through the composite floorunit, and it has been established that the preferred composite floorunit according to the invention is one with only the reinforcing mesh ofthe ceiling slab welded or brazed to the edge frame.

The supporting joists fulfill two different functions. Support for thesecond (top) slab must be to building regulation standards for thestrength and fire resistance of a load-bearing floor. That may beprovided by having the top slab simply rest on the joists, butpreferably the top slab is physically anchored to the joists by havingthe longitudinal top edges of the supporting joists embedded in thematerial of the top slab or by having anchorage members secured to thelongitudinal top edges of the supporting joists and embedded in thematerial of the top slab. Support for the first (bottom) slab may be tothe lower building regulation standard for the strength and fireresistance of a suspended ceiling, although according to the inventionit is possible to surpass that standard by a very considerable margin.The required support may be provided by having the longitudinal bottomedges of the supporting joists embedded in the material of the bottomslab or by having suspension members supported by the relevantsupporting joists and embedded in the material of the bottom slab.

The sound insulating material may wholly or partially fill the spacebetween the two cast slabs, which may be of the same or differentmaterials, and the same thickness as each other or of differentthicknesses. The top slab must be of a cement based material, such asconcrete. The bottom slab may be of a cement based material such asconcrete or a gypsum based material. Typical dimensions are that theindividual slabs may be from 50 to 100 mm thick with a separation offrom 150 to 300 mm. Preferably each slab has a thickness of about 65 mmand preferably the separation is about 225 mm. Other preferred oroptional features of the invention will be apparent from the followingdescription of the drawings.

DRAWINGS

FIG. 1 is a perspective view of a modular composite floor unit accordingto the invention, with a generally rectangular periphery;

FIG. 2 is a section taken along the line A-A of FIG. 1;

FIG. 2A is an enlarged section of the right hand end portion only ofFIG. 2;

FIG. 2B is a section through the cold-rolled sheet metal edge member ofFIG. 2A illustrating its method of construction;

FIG. 2C is a section through one of the joists of cold-rolled sheetmetal visible in FIG. 2A, illustrating its method of construction;

FIG. 3 is a section similar to that of FIG. 2A, but taken along the lineB-B of FIG. 1 through another of the cold-rolled sheet metal edgemembers;

FIG. 3A is a section through the cold-rolled sheet metal edge member ofFIG. 3, showing its method of construction;

FIG. 4 is a perspective view similar that of FIG. 1, but through themodular composite floor unit before the top layer of concrete is poured;

FIG. 4A is a section, greatly enlarged, through one of the reinforcingcross-straps visible in FIG. 4;

FIGS. 5 to 12 are enlarged sections, similar to that of FIG. 2A, througheight different embodiments of the invention, the sequence of Figuresbeing chosen to illustrate sound proofing considerations and thetechniques that can be used according to the invention to decrease thesound transmission in various wavebands through a series of modularcomposite floor units according to the invention;

FIG. 13 is a perspective view of a connector cradle for connecting thehollow beams of FIG. 12 to the top lattice of reinforcing rods or wires;

FIG. 13 a is a plan view of a sheet metal blank which can be folded toform an alternative connector cradle;

FIG. 13 b is a perspective view of the alternative connector cradlecreated by folding the blank of FIG. 13 a;

FIG. 14 is an enlarged section, similar to that of FIG. 2A, through aninth embodiments of the invention to illustrate another sound proofingtechnique that can be used according to the invention to decrease thesound transmission in various wavebands through a modular compositefloor unit according to the invention;

FIG. 15 is a perspective view of a connector hanger for connecting thelower row of hollow beams of FIG. 12 to the bottom lattice ofreinforcing rods or wires;

FIGS. 16 and 17 are enlarged sections through sheet metal edge membersof an edge frame of a modular composite floor unit according to theinvention, being the edge members of respectively a side and an end ofthe edge frame, and showing an alternative cold-rolled sheet metalprofile to those shown in FIGS. 2B and 3A;

FIG. 18 is a vertical. section though the junction between two floorunits according to the invention as installed in a building and two wallpanels of the building, to illustrate the support of the floor units bytheir out-turned flanges;

FIG. 19 is an enlarged section, similar to that of FIG. 2A, through apreferred embodiment of the invention;

FIG. 20 is a perspective view of the linking strut XX as used in FIG.19; and

FIG. 21 is a detail illustrating the construction of the metal edgemember of FIG. 19.

The modular composite floor unit of FIG. 1 comprises an edge frame 10made from cold-rolled sheet metal edge members brazed or welded togetherto form an accurately sized and proportioned edge shuttering for thefloor unit. Into that edge frame 10 is built up a composite floorassembly comprising two, spaced layers of poured reinforced concreteseparated by filler materials, as will be particularly described below.

The overall structure of the layered infill for the edge frame 10 isillustrated in FIG. 2. A bottom layer of poured concrete 12 and a toplayer of poured concrete 14 are separated by a space containing a layerof significantly less dense material such as lightweight walling blocks16. The walling blocks 16 are supported and separated by an array ofmutually parallel spaced joists 18 of cold-rolled sheet metal, theprecise shapes of which are better illustrated in FIGS. 2A and 2C. Theparallel spaced joists 18 are welded or brazed to the edge frame 10 attheir opposite ends, and L-section pieces of cold-rolled sheet metal 20and 22 are welded or brazed to the joists 18 and to the respective edgeframe members 24 which make up the edge frame 10, so as to providerunners for supporting the walling blocks 16.

The bottom slab of poured concrete 12 is poured around a reinforcinglattice of rods or wires 26 which are welded or brazed to the edge frame10 all around its periphery. A similar lattice of rods or wires 28provides reinforcement for the top layer of poured concrete 14. The factthat the rods or wires 28 are secured at their ends to the edge frame 10by welding or brazing has proved to be of enormous importance inproviding the fire resistance of the composite floor unit according tothe invention. The ceiling and floor slabs with those rods or wires asinternal reinforcement are joined integrally to the edge frame 10 in arow of such welded or brazed connections which preferably extendcompletely around the periphery of the composite floor unit. Furthermorethe anchorage of the cast slabs (ceiling and floor) to the edge frame 10can be considerably enhanced by allowing the unset material of the castslabs to flow into an around channel ends of C-shaped cold rolledsections of the edge frame 10, and preferably through apertures formedin the material of the C-shaped sections. For example the pouredconcrete of both the bottom and top concrete slabs extends throughapertures 25, 31 formed in the edge frame members 24 and 30 into theinternal cavities of the edge frame members 24 (FIG. 2A) and 30 (FIG. 3)so that the edge frame becomes an integral part of the composite floorunit. The mutually parallel spaced joists 18 which support the wallingblocks 16 are also embedded at their top and bottom edges in theconcrete of the bottom and top layers 12 and 14, which adds to thereinforcement of those concrete slabs and to the strength of thefinished floor unit.

The edge frame members 24 and 30 (FIGS. 2A and 3) could conceivably havethe same section as one another, although the corner joints of the edgeframe 10 would then have to be mitered. An alternative is illustrated inFIG. 3, in which the edge frame member 30 sits inside the generallyC-shaped section of the edge frame member 24 of FIG. 2A, with an endplate 32 being welded or brazed to the edge frame member 30 to bring itto the full height of the edge frame 10.

The method of construction of the modular composite floor unit of FIG. 1will now be described. First of all the edge frame 10 is built up infactory conditions. The edge frame members 24 and 30 can be laser-cut toa very high degree of accuracy. The edge frame members are thenpreferably set out on a factory floor or work bench and held in a jigwhile they are welded together to the precise size and proportions ofthe intended final floor unit. The joints 18 are welded or brazed toopposite edge frame members 30 while the edge frame 10 is held in thejig, and in this way the tolerances to which this work can be completedare vastly superior to those attainable on a building site. The firstlattice of rods or wires 26 is then welded or brazed into position. Eachof the lattices of rods or wires 26 and 28 may be a mesh of reinforcingrods or wires welded together into a square or rectangular grid ofcrossing rods or wires, such as the reinforcing mesh sold under theTrade Mark WELDMESH. If desired the top lattice 28 may be of heavierduty than the bottom lattice 26 because the bottom lattice 26 will inthe final multi-storey building become a part of the ceiling of the roombelow, and will therefore be subject to less strict buildingregulations. The securing of the bottom lattice 26 takes place allaround the periphery of the edge frame 10, and all of the assembly up toand including this stage is carried out with the floor unit underassembly being inverted, so that the lattice of rods or wires 26 iswelded to inturned flange portions 24A and to what will ultimatelybecome the lower surface 30 a of the edge frame members 24 and 30respectively. If desired, instead of a pre-welded mesh of rods or wiresthe lattice 26 could be of pre-tensioned wires 26 as described in GB0515075.0, the individual wires being drawn through apertures in theouter walls of the edge frame members 24 and 30, placed under tension,and then welded from the outside of the edge frame 10. Thispre-tensioning of the reinforcing lattice 26 can be repeated for thereinforcing lattice 28, and is possible because the edge frame 10 issecurely held in the jig on the work floor or work bench. Thepre-tensioning of the lattice is not, however, essential to the methodof construction of the composite floor unit according to the invention,and an alternative or additional method of using pre-tension to create avery stable edge frame structure is to incorporate diagonal cross-braces32 as illustrated in FIG. 4. Each cross-brace 32 is formed by unrollinga strip of sheet metal from a roll. If a slight crease 34 is formed inthe strip metal of the cross-brace 32, by cold-forming the strip into aslight apex along the line 34 as shown in FIG. 4A along most of itslength, then the tendency of the cross-brace strip to reform into a curlcan be largely or completely eliminated. Each cross-brace 32 is weldedor brazed at its ends to inturned flange portions of the edge frame 10,and preferably the cross-braces 32 when cold are under a slight tensionto ensure complete stability of the edge frame 10. The cross-braces 32may extend generally from corner to corner of the edge frame 10, or maybe arranged in any other pattern of triangulation.

When the welding of the edge frame 10, the lattice 26 and the optionalcross-braces 32 is complete, the edge frame 10 is turned over onto asmooth flat casting surface, ready for the casting of the bottom layer12 of poured concrete. The casting surface (not illustrated in thedrawings) may be any smooth flat surface coated with a concrete mouldrelease agent. It may, for example, be a flat metal surface such as thesmooth flat surface of a steel plate decking in the factory. Mirrorsteel may be used to provide an even smoother cast finish to theconcrete that is poured. Alternatively, the casting surface may betextured, to give an attractive textured appearance to the underside ofthe cast floor unit, which will become the ceiling of the room below inthe finished building. Clearly any texturing must be carefully regulatedso that it does not interfere with the mould release.

Alternatively, the casting surface may be covered with paper or fabricthat is preferably wetted, for example by spraying, with a bondingadhesive that causes it to adhere to the concrete that is poured intothe edge frame 10. That provides a paper or textured fabric finish tothe underside of the resulting floor unit, which provides the bestpossible paintable surface for ultimate ceiling decoration.

The concrete layer 12 may be poured as a single layer of liquidconcrete, or it may be built up in layers. For example a first layer,about 5 mm deep, of a grano gel coat may be poured first, followed by 25mm of C30 grade concrete. Concrete with a lightweight or porousaggregate is preferred, and the depth of the concrete is preferablymarginally above the level of the runners 20 and 22, as shown by abroken lead line 36 in FIG. 2A and in FIG. 3. It will be noted that theconcrete layer 12 flows through the apertures 25 into the edge cavities38 and 40 created by the shape of the edge frame members 24 and 30respectively (see FIGS. 2A and 3). Care should be taken to fill thosecavities completely for maximum strength.

While the poured concrete is still unset, rows of walling blocks 16 areplaced on the runners 20 and 22 and between adjacent parallel spacedjoists 18, completely to fill the floor space as defined by the edgeframe 10. The walling blocks 16 are preferably wetted beforeinstallation using a water-based bonding agent to ensure good adhesionto the concrete, and are preferably pressed into the unset concreteuntil they rest on the runners 20 and 22, so that the displaced concreteis pushed up between adjacent walling blocks, to provide better bondingwith the walling blocks 16. The top edges of the walling blocks 16create a generally planar surface, indicated in FIGS. 2A and 3 by thebroken lead line 42, for the pouring of the top layer of concrete 14.

Before the top layer of concrete 14 is poured, however, the secondlattice 28 of rods or wires is placed over the protruding tops of theparallel spaced joists 18, and welded to an inturned flange 44 of theedge frame members 24 and to an inturned flange 46 of the edge framemembers 30. Over the top of the lattice 28 there are then preferablywelded diagonal cross-braces 32 as illustrated in FIGS. 4 and 4A.

The top layer of poured concrete 14 is then poured over the tops of thewalling blocks 16. The concrete will settle down into any gaps betweenthe walling blocks, and will flow through apertures in the edge framemembers 24 as indicated by the shaded portions 48 of the joist members18 in FIGS. 2A and 2C, further to enhance the stability and rigidity ofthe resulting floor unit. Although the top layer of poured concrete 14will flow down and around the individual warning blocks 16, thatconcrete flow will not be sufficient to fill every void between the topand bottom layers of concrete 14 and 12, and simply for ease ofrepresentation, in FIGS. 2A and 3 the seepage of the top layer of pouredconcrete 14 down below the level of the tops of the walling blocks 16 isnot shown. The top layer of concrete 14 may be the same thickness asthat of the bottom layer 12, or may be a different thickness. In FIGS.2A and 3 the top layer is shown as being of a lesser thickness. Finally,the top surface of the top layer of poured concrete 14 is finished witha power float, to create a final finished floor unit which has a surfacefinish at least as smooth as the final screeded finish of conventionalbuilding techniques. That finish is certainly smooth and flat enough totake carpet, or tiles, or laminate flooring in the final building,without requiring a final top screed.

To lift the finished floor unit from the casting surface, liftingapertures or hooks or other handling formations (not shown) are formedaround the edge frame 10, and the finished floor unit can be lifted,after the concrete has set, by suitable handling equipment directly ontoa lorry or other transport vehicle, to the final site of the buildingunder erection. The accuracy of the dimensions of the floor unit, madeunder factory conditions, is such that it can be presented up topre-established mounting bolts or spigots on or in the building underconstruction, with a virtual guarantee of accurate alignment

Many modifications are possible to the method of construction describedabove. The function of the walling blocks 16, being less dense thanconcrete, is to reduce the overall weight of the floor unit. For thisreason, the above description refers by way of example to the use oflightweight walling blocks. Walling blocks made from a cinder or porousaggregate are highly suitable, such as those sold under the Trade MarkTHERMALITE™. The blocks 16 are provided for their sound insulationproperties and to create additional thickness to the floor unit withoutadding unduly to the overall weight, and a number of alternativematerials may therefore be used. For example, in place of walling blocksthere may be used blocks of expanded polystyrene, blocks of balsa wood,sheets of rockwool, sheets of fibreglass matting, or hollow mouldedplastic boxes. The blocks 16 could even be replaced by hollow boxes madefrom waxed cardboard. Plastic or cardboard boxes, when used, arepreferably filled with a sound absorbing material such as rockwool,fibreglass matting, shredded newspaper, paper mache, compressed straw,reclaimed particulate rubber or other lightweight products of therubbish recycling industry. Alternatively the longitudinal spacesbetween the bottom and top layers 12 and 14 of poured concrete can befilled by a lightweight particulate material such as chopped straw,pelleted newspaper waste, hollow balls or polystyrene beads. Boards ofwood or of a wood-based product such as plywood or oriented particleboard may then be placed over the fill material to create the generallyplanar surface 42 onto which the top layer of concrete 14 is to bepoured, and the remainder of the method of assembly is exactly asdescribed above. If the loose or particulate fill material iscompressible, or if it does not completely fill the space separating thetwo cast concrete slabs 12 and 14 of the finished floor unit, then itwill be preferred to incorporate runners (not illustrated) similar tothe runners 20 and 22 of FIG. 2A, to support the boards.

The complete floor units may be transported quite easily and safely andwith very little added protection required during transport, becausethey are protected from accidental edge damage by the edge frame 10which becomes an integral part of the construction.

It will be seen from FIGS. 1, 3, 3A and 4 that the edge frame members 30are formed with out-turned flanges 33 on their end plates 32. Similarout-turned flanges could if desired be formed on the edge frame members24 although they are not illustrated. The function of the out-turnedflanges is to support the floor unit during transportation and in thefinal building, where the floor unit can be laid in position to span anassembly of pre-assembled wall panels suspended initially by the flangesbefore being screwed, bolted, riveted or welded for final securement.

The top surface of the floor unit is as flat and smooth as the powerfloat operator can produce, which is a smoothness equal to that ofconventional floors screeded on-site. The under-surface is as smooth asthe casting surface on which the floor unit is made which, being infactory conditions, is a very high standard of smoothness. Alternativelyit may be paper-covered by casing onto paper as described above.Alternatively it may be textured, by casting onto a textured fabricwhich adheres to the underside of the floor unit after casting and whichthus establishes the texture of the resulting visible ceiling; or bycasting onto a textured casting surface.

The embodiment of FIGS. 1 to 4 utilises a sound insulating materialshown in FIGS. 2, 2A and 3 as walling blocks, which fill the full heightof the space between the bottom and top cast concrete slabs 12 and 14.That creates a floor unit which provides good acoustic insulation over arange of wavelengths, but for better sound insulation and also,incidentally, for better thermal insulation the sound insulatingmaterial should occupy less than the total space between the first andsecond concrete slabs. FIGS. 5 to 11 show seven alternative embodimentsof modular composite floor units according to the invention in which thesound insulation material is provided in a layer confined to the bottomportion of the space between the first and second concrete slabs, withan air gap above that insulation. In FIGS. 5 to 11 the same referencenumerals have been used wherever possible to those used in FIGS. 1 to 4,and the following description is limited to the differences between thedifferent embodiments.

The sound insulating material illustrated in FIGS. 5 to 11 isrepresented as a series of mats 50 of a sound insulating material suchas rockwool. It will be understood that any alternative particular soundinsulating material could be used, or any of the other materialsdiscussed earlier in this specification. In the embodiment of FIGS. 1 to4 the joists 18 have a J configuration as shown in FIG. 2C, the upturnedflange at the bottom of the J being used as a ledge on which to locatethe walling blocks 16. A simpler shape of joist 18A is shown in FIG. 5,being of C section. Advantageously the level to which the bottom layerof concrete 12 is to be poured may be marked on the joists 18A by meansof a scribe mark (not shown) or an aperture (not shown) punched throughthe vertical wall of the joists 18A before assembly, so that the bottomlayer of concrete 12 can be poured until it reaches the scribe marks orthe tops or bottoms of the punched apertures. As with the previousembodiment, the first and second lattices of reinforcing rods or wires26 and 28 are welded to the edge frame, as are the ends of the joists18A.

After the bottom concrete slab has been cast to the required depth, theinsulation mats 50 are laid between the joists, and boards 52 are placedon longitudinal supporting runners 54 which have been welded or brazedto the supporting joists. A similar runner 56 is spot welded or brazedto the inside of the edge frame member 24. The boards 52 provide a basefor the pouring of the second concrete slab 14 which is poured andfloat-finished as described earlier.

The air gap above the mats 50 in FIG. 5 reduces some of the soundtransmission between the top and bottom concrete slabs 14 and 12, and ofcourse enhances the thermal insulation of the composite floor unit ofFIG. 5. The longitudinal division of that air gap into relatively narrowhorizontal channels, by virtue of the joists 18A does act to reduce thesound transmission laterally along the floor unit, but the joists 18Athemselves provide a direct linkage and sound transmission path from onefloor slab to the other, and therefore provide a path for thetransmission of sound of certain frequencies. That sound transmissionpath can be broken by ensuring that the joists are divided into two.sub-groups of joists, namely joists 18B anchored at their ends in thetop concrete slab 14 as shown in FIG. 6, and joists 18C anchored attheir lower ends in the bottom concrete slab 12. In FIG. 6 those joists18B and 18C are shown as having a J section, the additional inturnedflange portion of the J section as opposed to the simple C section ofFIG. 5 providing the joists with increased stability and strengthagainst buckling along their unsupported edges. Nevertheless the joists18B and 18C of FIG. 6, which are shown arranged directly aligned oneabove the other, necessarily have a wall portion depending from the topslab of concrete 14 or a wall portion upstanding from the bottomconcrete slab 12 spanning less than half of the base between the twoconcrete slabs. The reinforcing effect of the joists 18B and 18C can beenhanced significantly by using wider joists as shown in FIG. 7, andstaggering them so that the joists 18B are offset on one side of thejoists 18C. By having a relatively small spacing between pairs ofadjacent joists as shown in FIG. 7, the turning moment transmitted fromone joist to the other at the outside edge of the edge frame ismaximised, for maximum strength. The number of joists 18B and 18C used,and their mutual spacing, is dependent on the width of the floor unitand the length which each joist has to span.

FIG. 8 shows an alternative arrangement of joists, with a pair of joists18C bedded in the bottom concrete slab 12 alternating with a pair ofjoists 18B embedded in the top concrete slab 14 across the width of thefloor unit. The advantage of this arrangement is that if desiredreinforcing straps 80, one only of which is shown in FIG. 8, can bewelded or brazed between the free edges of the pairs of adjacent joists,to strengthen the joist assembly and resist buckling.

It will be seen in each of FIGS. 6 to 8 that the runners 54 supportingthe boards 52 are welded or brazed to the joists 18B which areultimately to be embedded in the concrete of the top slab 14. Onealternative method of supporting the boards 52 is shown in FIG. 9.Blocks of expanded polystyrene 90 are placed on the top edges of thejoists 18C, and taller blocks of expanded polystyrene 92 are placed onthe inturned and upturned bottom edges of the joists 18B. The boards 52are simply balanced between adjacent pairs of blocks 90 or 92 prior topouring the concrete of the top layer 14. Expanded polystyrene is a verypoor conductor of sound, so that there is very little sound transmissionfrom the top concrete slab 14 to the bottom concrete slab 12 through theblocks 90 and 92, which do not play any structural role in the finalfloor unit once the concrete layer 14 has set. It will be understood ofcourse that a combination of polystyrene blocks and runners could beused. For example FIG. 10 shows a combination of the polystyrene blocks90 placed on the tops of the joists 18C, and runners 54 welded to thejoists 18B. FIG. 10 also illustrates how service ducts can beincorporated into the floor units of the invention. FIG. 10 illustratesa service duct 100, which may be for example a plastic conduit,extending laterally of the joist 18B and 18C. The duct 100 is suitablefor carrying electrical wiring either completely across the floor unitor from an outside edge to a mid position where it could be taken downthrough the ceiling, up through the floor, or simply turned at 90° torun parallel with the joists. The conduit 100 passes through holespunched in the joists 18B and 18C, but those holes are of differentsizes so that the conduit contacts and is supported by the joists 18B asillustrated in FIG. 10, whereas it makes no contact at all with thejoists 18C. Equally, the relative sizes of the holes punched in thejoists could be reversed so that the conduit is supported by the joists18C and makes no contact with the joists 18B. By avoiding contact withthe joists of one set, it can be ensured that sound transmission throughthe floor unit does not travel through the conduit 100.

FIG. 11 shows an alternative location for the service conduit 100,beneath the joists 18B and supported by holes punched in the joists 18C.The acoustic insulation mats 50 in FIG. 11 are shown as thicker thanthose in FIGS. 5 to 10, but that is principally because in thisembodiment the mats have to be wrapped up and over the conduit 100,giving them increased height along the section line of FIG. 11. Ofcourse, the acoustic insulation mats 50 of FIGS. 5 to 11 can be of anythickness, even occupying the full height between the bottom concreteslab 12 and the boards 52 on which the top concrete slab 14 is laid.

In FIGS. 5 to 8 the top slab 14 is cast over an array of discrete boards52. These boards 52 are supported on runners 54 secured to the joists18A or 18 b which support the top slab across its width. Use of separateboards 52, one between each pair of adjacent supporting joists 18A or18B, requires an additional step of cutting the individual boards 52 tosize and assembling them one by one between the joists and supported onthe runners 54. A preferred construction is to use a single board 52A asshown in FIG. 12. That board 52A is placed directly over the top ofjoists 18D which support the top slab 14 across its width. Those joists18D are shown in FIG. 12 as being hollow box section joists, althoughthey are made from cold-rolled sheet metal, as are the joists 18 of FIG.2C and the joists 18A of FIGS. 5 to 8. The very fact that the rigidboard 52A rests on the hollow section joists 18D means that the joists18D support the top slab 14 across its width, but that support isadvantageously considerably enhanced by a series of anchorage members 60which are screwed to the hollow joists 18D by means of self-tappingscrews 62 which pass through the solid board 52A. The anchorage members60 are cradle-shaped as shown in FIG. 13, each comprising a pair ofupright sides 64 upstanding from a flat base 66. Slots 68 are cut in thetop portions of the upright sides 64 to straddle the rods or wires ofthe enforcing lattice 28. When the anchorage member 60 is screwed to thehollow beams 18D through the rigid board 52A, this provides the totalsupport for the reinforcing lattice 28 both in the upward direction andthe lateral directions, as well as the main load bearing downwarddirection.

Each cradle 60 of FIG. 13 supports the reinforcing rods or wires of thelattice 28 running in one direction only, but different cradles 60 canbe oriented in mutually perpendicular directions so that together theysupport both the longitudinal and the lateral reinforcing rods or wiresof the lattice 28. Alternatively cradles 60 a as illustrated in FIGS. 13a and 13 b can be used. FIG. 13 a illustrates a sheet metal blank 60 bfrom which the cradle 60 a of FIG. 13 b can be formed by bending. Rowsof oval cut-outs in the blank 60 b are separated by relatively narrowmetal webs 63 so as to define fold lines enabling the sheet metal blankof FIG. 13 a to be easily bent by hand to the shape of FIG. 13 b. Apre-formed hole 65 is provided in the flange which becomes the base ofthe final cradle 60 a to receive the screw 62 of FIG. 12, and slots 68 aand 68 b receive the longitudinal and lateral reinforcing rodsrespectively of the reinforcing lattice 28. The slots 68 a and 68 b maybe at the same distance from the base as shown in FIG. 13 b, in whichcase the cradle 60 a is easily twisted along one of the fold lines inuse to bring the slots to the mutually different levels of thelongitudinal and lateral reinforcing rods; or the slots 68 a and 68 bmay be at mutually different heights to reflect the different levels ofthe longitudinal and lateral reinforcing rods.

FIG. 12 shows that the joists 18C supporting the bottom slab 12 areconstructed in the same way as those of FIG. 8, and connected togetherat intervals by lateral straps 80. The box section joists 18D areconsiderably stronger than the separate J-section joists 18C even whenthose joists 18C are joined together by straps 80, and an even strongerconstruction is therefore that shown in FIG. 14 in which the joistssupporting the bottom slap 12 are hollow box section joists 18E, similarto the hollow joists 18D supporting the top slab. The support betweenthe hollow joists 18E and the bottom slab 12 is provided by a series ofhangers 70 which are as shown in FIG. 15. Each hanger is a metal strapwhich passes over the joist from which it is suspended, and hangs downon opposite sides of that joist. Transverse slots in the lower ends ofthe hangers hook around and support the reinforcing rods or wires of thefirst lattice 26 to provide the necessary support across the width ofthe bottom slab 12.

It will be understood that instead of the metal of the strap hangers 70as shown in FIG. 15, the reinforcing lattice 26 for the bottom slabcould be supported from the hollow joists 18E by wires. Depending on thelength and diameter of the supporting wires, this will provide verylimited sound transmission between the hollow beams 18E and the lowerslab 12, which gives the possibility of a further embodiment (notillustrated) in which each transverse joist 18 can be formed as a hollowbox section joist that both supports the top slab as shown in FIG. 12and supports the bottom slab by means of connecting wires.

Although not illustrated, the hollow box section joists 18D and 18E ofFIGS. 12 and 14 can be wholly or partially filled by a sound-absorbingmaterial. Instead of the joists 18 of FIG. 12 and the joists 18D and 18Eof FIG. 14 being formed as hollow box sections as illustrated, animprovement in strength, as compared with the simple J-section joists18B and 18C of FIGS. 6 to 11, can be obtained by forming each joist ofFIG. 12 or 14 from two identical J-section joists placed back-to-backand secured together by spot-welding.

Another modification (not illustrated) is to place a layer of acousticrubber over the tops of the box sections 18D or the single orback-to-back J-sections, together possibly with an edge trim of acousticrubber between the cast concrete of the top slab 14 and the edge frame10. This gives a floating floor without detracting from the excellentrigidity and acoustic superiority of the modular floor units asdescribed and illustrated.

FIGS. 16 and 17 show an alternative section for the edge frame members24 and 30 of the edge frame 10. FIG. 16 shows that the out-turned flange148 at the top of the edge frame member 30 is slightly lower than thetop level of the concrete slab 14. As with FIG. 3A the edge frame member30 is made in two pieces, 30 a and 30 b, with an outer side plate 30Aforming that out-turned flange 148. FIG. 17 shows the out-turned flangesbeing level with the top of the top slab 14 of concrete. The way inwhich the lowered flange 148 of FIG. 16 is useful in the actualconstruction of buildings using floor units according to the inventionis illustrated in FIG. 18. 140 shows the top of a wall of the building,on which two floor units according to the invention are supported. FIG.18 shows one floor unit 142 to the right of the wall top 140, and onefloor unit 144 to the left. A rubber sheet 146 is placed over the topcap of the wall top 140 to reduce sound transmission through the finalbuilding, before the top floor units are placed in position, suspendedon their out-turned flanges 148. Self tapping screws or anchorage bolts150 are passed through downwardly extending anchorage plates 152 thatare welded or brazed to the side plates 32 of FIG. 16 to render theassembly rigid. The building is then ready to be increased in height byone further storey. If the flanges were not recessed below the top ofthe top floor slabs, there would be no positive line along which tolocate the next higher wall panel 154. By virtue of the recessed natureof the flanges 148, the next wall panel 154 can be positively located inthe shallow slot formed between adjacent floor units 142 and 144, and ispreferably protected from direct metal to metal contact with the flanges148 by another strip of rubber 156. If desired, filler pieces of rubber,plastic or metal can be placed between the top edges of the adjacentfloor units 142 and 144 and the wall. panel 154 being assembled intoposition, to shim the wall panel 154 into totally accurate alignment.

FIG. 18 also shows a pair of flexible hangers 158 of the wall panel 154,to which plasterboard panels 160 are attached in conventional manner. Anintumescent strip 162 is placed along the bottom of each set ofplasterboard panels 160, to fill the gap between the plasterboard and.the floor of the building being constructed.

It will be appreciated that the construction detail shown in FIG. 18reduces the amount of sound transmission vertically through thebuilding, so that the sound insulation properties of the floor units ofthe invention are put to very good effect.

The most remarkable advantage of all of the embodiments of compositefloor unit according to the invention as illustrated in FIGS. 1 to 18 ishowever their fire resistance. There is very little distortion of thefloor units in the event of a fire, because of the anchorage of the rodsor wires of the internal reinforcement of the two cast slabs to the edgeframe by welding or brazing, and because of the anchorage of the joists18, 18A, 18B, 18C, 18D and 18E of the various embodiments to the edgeframe by welding or brazing. The joists 18 to 18E of the variousillustrated embodiments described above have been cold-rolled steelprofiles. A further embodiment as illustrated in FIGS. 19 to 21 useshot-rolled metal section joists 18F which are of parallel flangedchannel profile. Alternative hot-rolled profiles would be I-beam orhot-rolled box section. A modular composite floor unit as described withreference to FIG. 19 was extensively tested in a fire resistance testand amazingly survived the test for the full 240 minutes of the BS 476Part 21: 1987, Clause 7 test.

Referring to FIGS. 19 to 21, the sides of the edge frame 30 areconstructed in two pieces as in FIG. 21. The parallel flanged channeljoists 18F are welded or brazed to the edge frame 30 at their ends.Hangers 70 shaped as in FIG. 16 straddle the joists 18F and support awelded mesh lattice 26 of reinforcing rods which will provide thereinforcement for the bottom cast slab 12 (the ceiling slab). All endsof the welded mesh lattice 26 are welded or brazed to an upturned andinturned flange portion of the edge frame 30. The welded meshreinforcing lattice 26 is therefore supported across its central portionby the hangers 70 and secured firmly to the edge frame 80 all around theperiphery. At this stage the ceiling slab 12 is cast, with thecement-based or gypsum-based casting material flowing into the edgechannel of the edge frame 30 all around the periphery of the floor unitand around the reinforcing lattice 26 across the centre. A bottomportion of each hanger 70 is encased in the cast slab 12 but the joists18F are above the level of the cast slab 12.

Insulation 50 such as high density rockwool insulation matting (forexample that sold under the Trade Mark BEAMCLAD) is then packed into thevoids above the cast slab and between the joists 18F, and one or moresolid boards 95 placed over the tops of the joists 18F. A very suitablematerial for those boards 95 is a fibre board impregnated with bitumen,as sold under the Trade Mark BITROC. If desired, additional support forthe boards 95 can be provided by first placing transverse beams 96between pairs of adjacent joists 18F at intervals along the length ofthe joists 18F. Each transverse beam 96, of which one is shown inperspective view in FIG. 20, comprises a box section support portion forthe solid board 95 and a pair of mounting plates 97, one at each end.The mounting plates 97 overlie the joists 18F as shown in FIG. 19, andcan if desired be secured in position by self-tapping screws (not shown)or by spot welds.

The solid boards 95 provide a base support for the upper slab ofconcrete 14 that is to be cast over the top of the composite floor unit.Before that concrete is poured, however, the lattice 28 of reinforcingrods is secured in position. Mesh anchorage members 60 or 60 a, asalready illustrated in FIG. 13 or in FIGS. 13 a and 13 b, are secured atintervals over each joist 18F and are secured to the joist 18F usingself-tapping screws 62 which pass through the solid board 95 and intothe joist. The lattice 28 of welded reinforcing rods is supported by theslots in the anchorage members 60 or 60 a and held spaced above theboards 95 across the width of the composite floor unit. The edge frame30 is itself made from two components 30 a and 30 b welded together asillustrated in FIG. 21.

Although not illustrated in FIG. 19, a sheet of polythene is laid overthe boards 95. The edges of the polythene sheet are trapped in theC-section component 30 b of the edge frame 30 by strips 30 c of expandedpolystyrene inserted into the C-section component 30 b between its upperand lower flanges. The cast floor slab 14 is therefore effectively afloating floor, supported across its width by the parallel flangedchannel joists 18F but isolated from the edge frame 30 by the expandedpolystyrene strips 30 c. The improvement in acoustic insulation of theresulting composite floor unit is remarkable. There is very little soundtransmission from the floor slab 14 to the framework of the building(for example to the wall top 140 of FIG. 18) because of the provision ofthe expanded polystyrene strips 30 c and the free floating nature of thefloor slab 14. Fire resistance could of course be improved by welding orbrazing the ends of the reinforcing rods of the lattice 28 of the floorslab 14 to the edge frame 30, just as the ends of the reinforcing rodsof the reinforcing lattice 26 of the ceiling slab are so welded orbrazed. That would however be at the expense of the sound insulationimprovement that is obtained by making the floor slab free-floating.Surprisingly, it has been found that the fire resistance is sooutstandingly good when only the bottom reinforcing lattice is welded orbrazed to the edge frame 30 that a similar edge connection of the topreinforcing lattice is unnecessary.

The floor unit as illustrated in FIGS. 19 to 21 was tested for fireresistance in accordance with British Standard 476: Part 21: 1987,clause 7. The unit was tested for its ability to comply with theperformance criteria for load-bearing capacity, structural integrity andthermal insulation. During the test the specimen floor unit being testedcarried a surface load of 2 KN/m² evenly distributed over its topsurface. Thermocouples were positioned over the top surface of the unitbeing tested, and the unit was suspended over a furnace which enabled itto be heated from below. The test was continued for four hours asspecified in BS476, and the specimen survived the full duration of thetest.

Even though the furnace temperature was raised to 1152° C. during thetest, the maximum temperature of the top surface of the floor unit evenafter 4 hours was only 68° C., indicating excellent thermal insulationbetween the top and bottom slabs of the floor unit. Structural integrityand load-bearing capability were maintained for the full 4 hours of thetest although there was a slight (but acceptable) bowing or sagging of apart of the bottom slab towards the end of the test. The specimen undertest still satisfied the test criteria for upper surface temperature,load-bearing capacity and structural integrity at the end of the 4-hourtest, which represents really astonishing performance characteristics,way beyond expectations which were for at most a 90-minute satisfactionof all of the test criteria.

In addition to the quite unpredictably high fire resistance of thespecimen floor being tested, that same floor unit had previously beensubjected to a test for acoustic insulation. It was found to be farsuperior to conventional solid floors and to conventional hollow floors.The excellent acoustic properties are thought to be a combination of thedense nature of the top and bottom slabs, the fact that those slabs areanchored all round their periphery to the edge frame by virtue of thewelded or brazed connections between the reinforcing lattice of rods orwires and the edge frame and between the joists and the edge frame, andthe less dense interior of the composite floor unit. The less denseinterior, provided by the rockwool 50 and the air gap over the rockwool,provides good acoustic insulation. The direct acoustic paths through thecomposite floor unit from the top surface to the bottom surface arelargely confined to the self-tapping screws 62 linking the top slab 14to the joists 18F, and the mesh hangers 70. By judicious spacing ofthose hangers 70 the composite floor unit of the invention achieves, ina total thickness or depth of less than 300 mm for the floor unit, alevel of acoustic insulation that might be expected of a conventionalfloor unit at least twice as thick.

1. A modular composite floor unit for an above-ground-level floor of abuilding, comprising: an edge frame made from cold-rolled sheet metaledge members welded or brazed together to form an accurately sized andproportioned edge shuttering for the floor unit: a cast cement-based orgypsum-based ceiling slab cast within the edge frame and encasing afirst lattice of reinforcing rods or wires; and a cast cement-basedflooring slab spaced from the ceiling slab and cast within the edgeframe, encasing a second lattice of reinforcing rods or wires which arewelded or brazed at their ends to opposite edge members of the edgeframe; each of the ceiling and flooring slabs being supported across itswidth by an array of mutually parallel spaced metal joists which extendacross the floor unit between the cast slabs and are welded or brazed attheir opposite ends to opposite edge members of the edge frame; and thespace between the ceiling and flooring slabs containing a soundinsulating material of lower density than that of the cast ceiling andfloor slabs.
 2. A floor unit according to claim 1, wherein the soundinsulating material completely or substantially completely fills theinter-joist space between the cast ceiling and floor slabs.
 3. A floorunit according to claim 2, wherein the sound insulating materialcomprises an array of blocks of lower density than that of the materialof the cast ceiling and floor slabs.
 4. A floor unit according to claim3, wherein the blocks are blocks of a cinder-based or porousaggregate-based cement walling block material, expanded polystyrene,rockwool, compressed straw or balsa wood; or are plastic or cardboardboxes filled with loose particulate material such as rockwool, shreddednewspaper, paper mache, chopped straw, glass fibre matting or reclaimedparticulate rubber.
 5. A floor unit according to claim 3, wherein thecast floor slab has been cast directly over the tops of the array ofblocks and has been allowed to flow around and between the blocks of thearray.
 6. A floor unit according to claim 2, wherein thesound-insulating material comprises a layer of a lightweight soundabsorbing material laid over the top of the cast ceiling slab andbetween the joists, and a solid board or an array of solid boards placedover the sound absorbing material, the board or boards being supportedby the sound absorbing material or by the joists.
 7. A floor unitaccording to claim 6, wherein the cast floor slab has been cast over thetop of the solid board or boards.
 8. A floor unit according to claim 1,wherein the sound insulating material only partially fills the spacebetween the cast ceiling and floor slabs.
 9. A floor unit according toclaim 8, wherein the sound insulating material comprises a layer oflightweight sound absorbing material laid over the top of the castceiling slab and between the joists, and a solid board or an array ofsolid boards supported by the joists at a level spaced from the top ofthe layer of sound-absorbing material.
 10. A floor unit according toclaim 9, wherein the cast floor slab has been cast over the top of thesolid board or boards.
 11. A floor unit according to claim 1, whereineach joist has one longitudinal edge embedded in the material of thecast ceiling slab and an opposite longitudinal edge embedded in thematerial of the cast floor slab.
 12. A floor unit according to claim 11,wherein the joists are made from cold-rolled C-section steel.
 13. Afloor unit according to claim 1, wherein each joist has one longitudinaledge embedded in the material of one of the cast slabs and an oppositelongitudinal edge in the space between the cast slabs, in an alternatingsequence of joists or pairs of joists across the floor unit.
 14. A floorunit according to claim 13, wherein the joists are made from cold-rolledJ-section steel, the inturned longitudinal edge of each section beingthat which is located in the space between the cast slabs.
 15. A floorunit according to claim 13, wherein the joists having a longitudinaledge embedded in the material of the cast ceiling slab are offsetlaterally from those having a longitudinal edge embedded in the materialof the cast floor slab, the wall portions of the respective joistsextending more than half way across the space between the cast ceilingand floor slabs.
 16. A floor unit according to claim 15, wherein pairsof adjacent joists, one having an edge embedded in the material of thecast ceiling slab and the other having an edge embedded in the materialof the cast floor slab, are closely adjacent one another and separatedby a greater distance from adjacent similar pairs of joists.
 17. A floorunit according to claim 16, wherein free edges of adjacent joists havingedges embedded in the material of the same cast slab, those free edgesbeing the edges located in the space between the slabs, are joinedtogether at intervals along the length of the joists by straps whichtransfer buckling loads between the joists.
 18. A floor unit accordingto claim 10, wherein the joists are hollow box section joists or hotrolled parallel flanged channel joists.
 19. A floor unit according toclaim 18, wherein the cast ceiling slab is supported by the joistsacross its width by hangers suspended from the joists or from some ofthe joists and supporting the first lattice of reinforcing rods or wiresacross the width of the cast ceiling slab.
 20. A floor unit according toclaim 19, wherein the hangers are wire hangers.
 21. A floor unitaccording to claim 19, wherein the hangers are metal straps which passover the joists from which they are suspended and hang down on oppositesides of those joists, having transverse slots in lower ends of thehangers which hook around and support the reinforcing rods or wires ofthe first lattice.
 22. A floor unit according to claim 18, wherein thecast floor slab is supported by the joists across its width by beingcast on a solid board resting directly on the top of those joists.
 23. Afloor unit according to claim 22, wherein the cast floor slab isanchored to the joists which support it across its width by an array ofanchorage members which are connected to the second lattice ofsupporting rods or wires and are screwed to the joists which support thecast floor slab through the solid board.
 24. A floor unit according toclaim 23, wherein each of the box section joists supports both the castceiling and floor slabs.
 25. A floor unit according to claim 23, whereinalternate ones of the box section joists across the width of the floorunit support the cast ceiling slab and intermediate ones of the boxsection joists support the cast floor slab.
 26. A floor unit accordingto claim 18, wherein the box section joists contain a sound-absorbingmaterial.
 27. A floor unit according to claim 10, wherein embedded inthe cast ceiling slab and/or the cast floor slab are reinforcingdiagonal cross-struts welded at their ends to the edge frame.
 28. Afloor unit according to claim 27, wherein the reinforcing diagonalcross-struts are made from ribbons of sheet metal that have beenunrolled from a roll and prevented from curling by imparting alongitudinal crease thereto.
 29. A floor unit according to claim 10,wherein each of the cast ceiling and floor slabs has a thickness of from50 to 100 mm, and the space between the cast ceiling and floor slabs isfrom 150 to 300 mm.
 30. A floor unit according to claim 29, wherein eachof the cast ceiling and floor slabs has a thickness of about 65 mm. 31.A floor unit according to claim 29, wherein the space between the castceiling and floor slabs is about 225 mm.
 32. A floor unit according toclaim 10, wherein the surface finish of the underside of the castceiling slab is a paper or fabric material that has been laid over thecasting surface on which the ceiling slab has been cast.
 33. A floorunit according to claim 10, wherein the surface finish of the undersideof the cast ceiling slab is the surface finish of a board or plate thathas been covered with a mould release agent before casting the ceilingslab.
 34. A floor unit according to claim 10, wherein the surface finishof the top surface of the cast ceiling slab is a power float finish. 35.A method for the manufacture of a floor unit, which comprises: formingan edge frame by welding or brazing together cold-rolled sheet metaledge members; welding or brazing to the edge frame an array of mutuallyparallel spaced metal joists; welding or brazing to the edge frame theends of the reinforcing rods or wires of the first lattice; casting theceiling slab by pouring wet concrete or gypsum-based plaster into theshuttering created by the edge frame, to a depth sufficient to encase afirst inturned lip of the edge frame, to encase the first lattice ofreinforcing rods or wires, and to embed lower longitudinal edges of someor all of the parallel spaced joists of cold-rolled sheet metal or ofhangers suspended from those joists; placing between the parallel spacedjoists of cold-rolled sheet metal the sound insulating material of lowerdensity than the concrete of the cast slabs and optionally the array ofsolid boards to form a base for the cast ceiling slab; welding orbrazing to the edge frame the ends of the reinforcing rods or wires ofthe second lattice; pouring wet concrete over the top layer of soundinsulating material or over the tops of the solid boards to a depthsufficient to encase a second inturned lip of the edge frame, to encasethe second lattice of reinforcing rods or wires and optionally to embedupper longitudinal edges of some or all of the parallel spaced joists ofcold-rolled sheet metal; and providing a smooth surface finish to thetop of the cast floor slab using a power float.